Three-terminal junction transistors have application to numerous electronic devices, including digital circuits, analog-to-digital converters, digital-to-analog converters, mixed-signal circuit, fiber-optic receivers, and microwave amplifiers.
A three-terminal transistor has an emitter, a base, and a collector with the base controlling the current transport from the emitter to the collector. Such a transistor can operate as a unipolar transistor having one predominant carrier transport type or as a bipolar transistor having two carrier transport types.
A unipolar transistor is typically designed to have electrons as the predominant carrier type. In addition, unipolar transistor device structure can be designed so that the carriers are hot electrons within the base of the transistor. If the base is thin, the carriers can have ballistic transport through the base. The unipolar transistor is typically operated primarily in the voltage-control mode with the injection of electrons from the emitter into the base controlled by varying the voltage difference applied between the emitter and base electrodes, with some portion of the electrons injected into the base transporting through the base and being collected by the collector.
The three-terminal transistor can also operate in a bipolar-mode where the injection of one carrier type (holes) into the base modulates the injection of a second carrier type (electrons) from the emitter into the base with some portion of the electron injected into the base transporting through the base and being collected in the collector. The bipolar transistor is typically operated primarily in the current-control mode.
The capabilities of these circuits are often rated by the speed of operation of the circuits. The speed of operation of the circuit type that utilizes three-terminal transistors is often dominated by the transit time of carriers through the base layer of the three-terminal transistor. Thus, it is often useful to use a thin base layer to achieve a fast carrier transit through the base layer.
One measure of the frequency performance capability of a three-terminal transistor is the transition frequency (cutoff frequency), fT, which is strongly dependent on the transit time of carrier through the base of a transistor. The maximum oscillation frequency Fmax for a bipolar transistor is approximately described by the equation
      F    max    =            (              fT                  8          ⁢          π          ⁢                                          ⁢          RbCcb                    )              1      /      2      where Rb is the base resistance, Ccb is the collector base capacitance, and fT is the cutoff frequency. Thus, the maximum frequency of oscillation is inversely proportional to the base resistance Rb. See Bart Van Zeghbroeck, “Chapter 5: Bipolar Junction Transistors,” Principles of Semiconductor Devices, available online at http://ecee.colorado.edu/˜bart/book/book/content5.htm (downloaded Jun. 15, 2011).
It has been known for some time that a hot electron transistor (HET), and especially a ballistic transistor, can potentially be operated at frequencies in excess of those achievable with conventional (diffusive) transistors. T. E. Bell, “The Quest for Ballistic Action,” IEEE Spectrum, February 1986, pp. 36-38. Various types of hot electron transistors (HET) have been proposed. See L. F. Eastman, “Ballistic Electrons in Compound Semiconductors,” IEEE Spectrum, February 1986, pp. 42-45; A. F. J. Levi et al., “Injected-Hot-Electron Transport in GaAs,” Phys. Rev. Lett. 55(19), pp. 2071-2073 (1985); M. Heiblum et al., “Direct Observation of Ballistic Transport in GaAs,” Phys. Rev. Lett. 55(19), pp. 2200-2203 (1985); and M. I. Nathan et al., “A Gallium Arsenide Ballistic Transistor,” IEEE Spectrum, February 1986, pp. 45-47.
One type of HET transistor has a thin P-type doped emitter material layer in the emitter transition layer at the emitter/base interface that implements a thermionic emission injection structure, also known as a planar doped barrier or a camel barrier, and a thin P-type collector material layer in the collector transition layer at the collector/base interface that implements a collector barrier, see J. R. Hayes et al., “Hot Electron Spectroscopy,” Electron. Lett. 20(21), pp. 851-852 (1984); J. M. Shannon, “Calculated performance of monolithic hot-electron transistors,” IEE Proceedings 128(4), pp. 134-140 (1981).
Another type of transistor uses tunnel injection and comprises emitter, base, and collector, with an appropriately shaped potential barrier between emitter and base, and a second barrier between base and collector. This second type, which is referred to as a tunneling hot electron transfer amplifier (THETA), differs from the first type in having tunnel injection into the base. See M. Heiblum, “Tunneling Hot Electron Transfer Amplifiers (Theta): Amplifiers Operating Up To The Infrared,” Solid-State Electronics 24, pp. 343-366 (1981).
Both of the above types are unipolar, although bipolar HETs have also been proposed.
Other types of bipolar transistors with thin base layers include a Bipolar Quantum Resonant Tunneling Transistor. See A. C. Seabaugh et al., “Pseudomorphic Bipolar Quantum Resonant-Tunneling Transistor,” IEEE Transaction on Electron Devices, Vol. 36, pp. 2328-2333; and S. Mil'shtein et al., “Bipolar Transistor with Quantum Well Base,” Microelectronics Journal 39, pp. 631-634 (2008). This device structure utilizes tunneling potential barriers at the emitter/base interface and the collector/base interface to establish a potential well with distinct allowed energy levels in the base layer. Electrons tunnel through the tunnel barrier at the emitter/base interface, enter allowed energy levels in the base, transit through the base, and then tunnel through the tunnel barrier at the collector/base interface and then enter the collector.
The flow of electrons from emitter to base is controlled in both types by varying the emitter/base barrier potential by means of an applied voltage Veb. The flow of electrons from the base to the collector can is made favorable by means of an externally applied voltage Vbc between base and collector. Under normal operating conditions, Vbc reverse biases the base/collector junction. Electrons injected from the emitter into the base have energy substantially greater than the thermal energy of the ambient electrons in the base. These “hot” electrons ideally traverse the base without undergoing significant scattering. If the barrier at the base/collector interface is lower than the hot electron energy, then some of the hot electrons can cross the barrier, be transmitted through the depletion region of the collector, and enter the sea of conduction electrons in the collector.
One of the critical parameters for transistors is the base resistance. A low base resistance is important to achieve a high maximum frequency of oscillation, fmax.
Graphene is a monolayer of conjugated sp2 bonded carbon atoms tightly packed into a two-dimensional (2D) hexagonal lattice. One of the primary advantages of graphene is that it has extremely high intrinsic carrier (electron and hole) mobility and thus has extremely high electric conductivity. Graphene has the potential to have the highest conductivity and lowest resistivity of any material, with a conductivity even higher than that of silver. See Chen et al., “Intrinsic and extrinsic performance limits of graphene devices on SiO2,” Nature Nanotechnology 3, 206-209 (2008).
For example, experimental results indicate that the resistivity of a single sheet of graphene approximately 3 angstrom thick grown on the silicon face of SiC has a sheet resistance on the order of 750 ohm/square to 1000 ohms/square, while a graphene sheet grown on the surface of copper can have a sheet resistance of approximately 1200 to 1500 ohms/square. In some cases, the sheet resistance of few sheets of graphene can be even less, as little as 100 ohms/square. Chen et al., supra.
The high electrical conductivity of graphene allows the use of a extremely thin graphene material base layer, even one comprising only a single graphene sheets and having a thickness of approximately 0.28 nm for a single sheet of graphene.
Use of such a thin graphene base layer reduces the transit time of electrons through the base layer and also reduces the energy loss of hot electrons in transiting the thin graphene base material. In addition, the high velocity of electrons in the graphene material can lower the base transit time. Thus, the semiconductor device with a graphene material base layer can have high fT and high fmax.